Communication devices need energy for operation, and in many cases this energy comes from electrical batteries. Batteries enable communication devices to be compact and mobile, operate in remote areas, independent on power lines or power generators, reduce costs, etc. However, batteries obtain a limited capacity of energy, thus a limited operational life (time), then should be replaced or recharged.
In order to save battery life, communication devices are often configured to periodically switch over to non active operational modes, and stay at low power consumption as low and as long as possible. In “low power consumption mode”, also known as “power save” or “power saving” or “power down” or “standby” or “sleep” mode, or similarly, the device turns off much of its circuitry, normally except of some low power consumption parts which are kept alive to ensure proper recovery or “wake-up”.
However, in power saving mode, normally nor transmitter or receiver is turned on, so a communication device in this mode is temporarily disconnected from the network and from its peers. Then, if unaware peers try to contact it, they will waste battery power, jam other receivers and generate noise, in vain. Similarly, if a device wakes up and starts transmitting unsynchronized with other peers, it might overlap other transmissions, consequently reduce communication success probability, jam other receivers and waste its battery as well.
A more complex scenario takes place when both devices, wishing to communicate data with each other, periodically stay in power save mode. Obviously, only when both devices are active, there is a chance to communicate in between, so in order to increase communication success probability it is desirable that active time slots of both devices overlap. Clearly, this requires time synchronization between said communication devices. Furthermore, such synchronization becomes paramount as the duty cycle of active time slots decreases.
Therefore, time synchronization among communication devices, which is usually an important issue, is particularly important when power saving modes are often been applied.
Another way to save energy and costs in communication devices is to employ low frequency, low resolution or low accuracy clocks. State of the art communication devices usually obtain built in clocks that administer the device operational timing. Such clocks are typically based on crystal oscillators, similar to those found in digital wrist watches. Accurate and fast clocks are required by communication devices to better and faster synchronize and less interfere with each other, among other advantages. However, accurate and fast clocks are relatively power consuming and expensive, for several reasons. In CMOS, a most popular technology for low power integrated circuits, power consumption depends on the switching frequency, thus faster oscillators consume more power. Further, though crystal oscillators made a breakthrough in time measuring, when introduced, they still obtain accuracy limitations, due to component parameters tolerance, temperature and aging effects. Some techniques are practiced in the art to improve oscillators' accuracy, as component screening (pre assembly) and additional on board or on chip temperature compensating circuitry, yet these methods add costs and power consumption. Another approach practiced in the present art is to employ several time references in a single communication device, e.g. a TCXO (Temperature Compensated Crystal Oscillator) for accurate active operation and a less accurate LPO (Low Power Oscillator) for power save mode. This obviously adds costs. Some communication devices do not employ an internal clock at all and operate asynchronously, saving clock power and costs, however this approach is problematic as traffic increases and multiple devices try to access the same communications medium, simultaneously.
When using low accuracy clocks, a discrepancy of some seconds (or tens or even hundreds of second) per year among a plurality of such communication devices, might be expected (1 part per million [ppm] is equivalent to approximately 30 seconds per year). For burst transmitting data applications, where a typical transmission can take less than a second, such a clock discrepancy matters.
U.S. Pat. No. 6,473,607 to Shohara et al. discloses a communication device with a self-calibrating sleep timer, with a dual mode timer that extends battery life. A controller schedules the timer to power down all idle components of the device in a power saving sleep mode to conserve battery power. During active mode the timer uses a reference oscillator with a relatively high frequency, but during sleep mode when only the timer is powered on, a much lower frequency sleep oscillator is used to maintain the lowest possible level of power consumption within the timer itself. The timer has provision for automatic temperature calibration to compensate for timing inaccuracies inherent to the low-power-low-frequency crystal oscillator used for the sleep mode. The resultant improvement in timer accuracy during sleep mode eliminates the need for an initial reacquisition period following wake up in active mode, thereby reducing battery drain in active mode as well. Still, such approach is relatively expensive, since it employs two sets of oscillators (or at least auxiliary circuitry for temperature compensation), and also can't avoid the relatively high power consumption concerned with the operation of the high frequency and high accuracy oscillator in active mode.
Further, in order to compensate for low accuracy and inconsistency of clocks among networked communication devices, the present art teaches many methods for time synchronization among devices that employ discrepant clocks, usually based on a signal sent from one device to another or broadcast from point to multipoint.
For example, a “master” device may transmit a synchronization signal plus time stamp (time tag) referring to that signal, which is received and adopted by a “slave” device, even if this time stamp is not accurate by universal standards. Alternatively, a precise clock signal, e.g. such which is generated by an atomic resonator, may be distributed and adopted by networked devices, in order to adjust their low cost and low accurate clocks. Further, prior art teaches methods to refine clock synchronization to compensate for propagation path delay of the synchronization message traveling between transmitter and receiver. Such methods are taught by the following U.S. patents.
U.S. Pat. No. 7,277,737 to Vollmer et al. discloses a method for power saving operation of communication terminals in a wireless packet switching communication system, wherein a master station sends synchronization information to communication terminals in an announcement channel, and receives messages in return, said terminals analyze the synchronization information and accordingly correct respective time bases. Communication terminals operate in one of two or optionally three modes: active, standby (optional) or sleep. In active mode—terminals monitor each announcement; in standby mode—at least one component of terminals is deactivated and the announcement channel is periodically monitored; in sleep mode—at least one more component is deactivated, reactivation time is longer and announcement channel is monitored less frequent.
U.S. Pat. No. 7,239,626 to Kandala et al. discloses a method of synchronizing clocks in the stations of ad hoc and infrastructure networks by providing a time stamp field in a header; reading the header by all stations in a network; extracting time stamp information from the header by each station in the network as time information; sending extracted time information to a station clock; adjusting the station clock as a function of the extracted time information; and providing a Delay Locked Loop (DLL) having a comparator for receiving the time stamp information and a low-pass filter having a long time-constant for adjusting the station clock in a gradual manner.
An efficient time synchronization method is applied in the US GPS (Global Positioning System). GPS requires a precise clock for positioning determination (1 microsecond in clock accuracy is roughly equivalent to 300 meters in position accuracy), yet precise clocks are large and power consuming and expensive, impractical for end user devices. Alternatively, the GPS obtains very few precise clocks installed in some terrestrial base stations, less precise clocks onboard 30 satellites, and low cost/low power/low accurate clocks embedded in millions of end user's terminals, employing an efficient method to synchronize less accurate clocks to more accurate ones.
However, when there is no communication between two devices, due to distance or inactivity, for example, such present art methods become impossible.
As more communication devices share the same medium, e.g. frequency channel, collision likelihood increases. As already indicated, collisions among simultaneous transmissions can be decreased by synchronizing transmitters. One very popular method for that purpose is Time Division Multiple Access (TDMA), particularly employed in cellular networks. TDMA allows several users to share the same transmission medium by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using its own timeslot. This allows multiple stations to share the same radio frequency channel while using only the part of its bandwidth. TDMA is used in digital cellular systems such as GSM, PDC and iDEN, as well as cordless standards as DECT.
Yet, TDMA requires precise time synchronization among communication devices, which is very difficult to achieve with low cost remote devices that sleep most of the time, rarely communicate and do not share a common clock. Such quasi sleepy low traffic devices may be part of many systems, applied to various fields, such as transportation, surveillance, reconnaissance, remote sensing, telemedicine and utility (gas, electricity, water) consumption. The latter application concerns with a multitude of low power radios attached to utility meters, deployed in large areas, part of a system and method known as Automatic Meter Reading (AMR). Such systems are very popular and already deployed by tens and even hundreds of millions of units worldwide.
U.S. Pat. No. 7,050,420 to Findikli discloses a system for maintaining synchronization between multiple asynchronous communication links, by carefully monitoring several network clocks.
This method also requires receiving and analyzing transmissions of other communication devices, thus cannot be done out of range and inefficient when devices are dormant most of the time.
U.S. Patent Application 20030129949 to Selektor discloses a system for remote control communication including secure synchronization, comprising communication devices comprising a data transceiver and a synchronization counter, wherein one transceiver transmits a synchronization counter value to the other transceiver to establish synchronization, wherein the synchronization process may use different radios, independent on said transceivers, even on different frequency bands.
Obviously, this method depends on the communications range of both links, for data and for synchronization.
The petroleum industry is increasingly concerned with ‘measuring-while-drilling’ (MWD) methods that allow early access to information about the geologic and fluid conditions surrounding the borehole as the drilling progresses. For that purpose, a seismic receiver package incorporating seismic and other sensors combined with processing means is capable of acquiring seismic data while drilling. This device is normally battery powered and transmits the acquired data wirelessly to the surface. However, as the downhole clock drifts relative to the master clock at the surface, significant errors accumulate in the seismic travel time measurements. The following U.S. patent and U.S. patent application teach a method to synchronize the downhole clock to the surface master clock.
U.S. Pat. No. 6,002,640 to Harmon discloses a seismic data acquisition system that utilizes a series of nearly identical seismic shots (SISS) to synchronize and to communicate with novel data acquisition units (NDAU) located in the field. Each SISS seismic shot is carefully timed to provide synchronization to each NDAU, and to allow the NDAU to correct for the time drift of its internal clock.
U.S. Patent Application 20060203614 to Harmon discloses a vertical seismic profiling method utilizing seismic communication and synchronization, utilizing seismic shots as a means for synchronizing a downhole clock in the VSP receiver to a master clock at the surface.
The last inventions deal only with two clocks, a master and a slave, and are based on a predefined time interval between consecutive signals (shots). Furthermore, the synchronizing signals are seismic in nature, compatible to the receiver been synchronized.
The present art teaches also asynchronous methods for multiple access of a shared communication medium, enabling communications among devices, independent (or less dependent) on clocks' consistency. A known asynchronous communications method is ALOHA.
ALOHA is a simple communications scheme in which each transmitter in a network sends data whenever there is a frame to send. If the frame is successfully received, the next frame is sent. If the frame fails to be received at the destination, it is sent again. This protocol was originally developed at the University of Hawaii for use with satellite communication systems in the Pacific, among remote devices that employ different reference clocks. An improvement to the original Aloha protocol was Slotted Aloha, which introduced discrete timeslots and doubled the maximum Aloha throughput. Yet, in order to efficiently employ Slotted Aloha, stations need to be in time synchronization, which is not easy to achieve if these distributed stations use different clocks that drift one compared to another. Furthermore, such method is particularly problematic when these stations sleep most of the time.
The present art methods described above have not yet provided satisfactory solutions to the problem of time synchronization among distributed wireless devices which are most of the time dormant, or sometimes out of communications range.
It is an object of the present invention to provide a system and method for time synchronization among distributed communication devices, specifically wireless.
It is another object of the present invention to provide a system and method for time synchronization among distributed communication devices that are most of the time in power save mode.
It is also an object of the present invention to provide a system and a method for time synchronization among distributed wireless communication devices which are beyond communications range.
It is yet another object of the present invention to provide a system and method for time synchronization among distributed communication devices limited in power consumption.
It is also an object of the present invention to provide a system and a method for time synchronization among distributed communication devices which are part of a local area or mesh or ad-hoc network.
It is still another object of the present invention to provide a system and method for time synchronization among distributed communication devices that present relatively low costs.
It is yet an object of the present invention to provide a system and method for time synchronization among distributed communication devices which could be applied to one at least of the following fields: transportation; surveillance; reconnaissance; remote sensing; telemedicine; and specifically Automatic Meter Reading (AMR) of utility (gas, electricity, water) consumption.
Other objects and advantages of the invention will become apparent as the description proceeds.